Synthetic polymeric fibers that have physical and morphological characteristics generally similar to pulp fibers produced from natural woods have been known for approximately 10 years. Examples of such fibers are the synthetic wood pulp fibers formed of polyethylene that are sold by Crown Zellerbach under the trademark SWP.
Various methods of making synthetic wood pulp fibers are known, including (1) solution polymerization accompanied by stirring, (2) dissolving a preformed polymer and subjecting the solution to an anti-solvent, or (3) forming the polymer at the interface between liquid layers, with localized stirring provided to pull the polymers thus formed into fibrillated forms. Examples of methods of producing synthetic wood pulp fibers are disclosed in U.S. Pat. Nos. 3,560,318; 3,081,519; 3,003,912; 3,068,527; and 3,290,207; South African Pat. No. 697,432; and United Kingdom Pat. No. 1,102,342; and Netherlands patent application No. A132/48-7313173.
As used in this specification and the appended claims, the term "synthetic wood pulp fibers" means synthetic, water dispersible, thermoplastic, elongated, supple, randomly bent, polymeric fibers or fibrils generally similar in length and denier to conventional wood pulp fibers produced from naurally occurring woods. Each such "synthetic wood pulp fiber" is of irregular cross sectional shape measured at any given point along its length, and in addition is nonuniform in cross section along its length. The predominant shape of the fibers is usally rather ribbon-like.
Though synthetic wood pulp fibers are similar in length and denier to conventional wood pulp fibers, their uniformity is better and their size and shape consistency greater. Whereas, conventional wood pulp fibers have a length which varies from 0.5 mm to 5.0 mm and a coarseness of between ten and twenty decigrex synthetic wood pulp fibers have a length of from one to four millimeters and a coarseness of between three and ten decigrex (as disclosed, for example, in the section headed "Fiber Dimensions" in the September, 1974 publication by Crown Zellerbach entitled SWP).
Comparing some of the other properties of synthetic wood pulp fibers with conventional wood pulp: a conventional wood pulp fiber has from 1 to 5 times the breaking stress of synthetic wood pulp; whereas, the synthetic wood pulp fiber elongates at rupture 3 to 5 times as much as a conventional wood pulp fiber; hence, the overall toughness of a synthetic wood pulp fiber is similar to or even greater than that of a conventional wood pulp fiber (as disclosed, for example, in the section headed "Single Fiber Stress-Strain Behavior" in the September, 1974 publication by Crown Zellerbach entitled SWP).
The present invention utilizes synthetic wood pulp fibers in a high loft, low density, nonwoven fibrous material such as an air-laid web or fabric. Nonwoven materials are structures which consist of an assemblage or web of irregularly arranged fibers, joined randomly or more or less systematically by mechanical, chemical or other means. These materials are well known in the art, having gained considerable prominence within the last twenty years or so in the consumer market, the industrial commercial market and the hospital field. For example, nonwoven materials are becoming increasingly important in the textile and related fields, one reason being because of their low cost of manufacture for a given coverage as compared to the cost of more conventional textile fabrics formed by weaving, knitting or felting. Typical of their use is the production of hospital caps, dental bibs, eye pads, dress shields, shoe liners, shoulder pads, skirts, hand towels, handkerchiefs, tapes, bags, table napkins, curtains, draperies, absorbent batts, diaper facings, underpads, hospital drapes, and the like. Generally speaking, nonwoven materials are available today in a wide range of fabric weights of from as little as about 100 grains/sq. yd. to as much as about 4,000 grains/sq. yd. or even higher.
A number of processes and types of apparatus are known for producing nonwoven materials. These include (1) mechanical techniques (e.g., carding or garnetting), (2) wet laying techniques (e.g., inclined wire paper apparatus, cylinder paper apparatus, etc.) and (3) air-laying techniques. The high loft, low density, nonwoven materials such as webs or fabrics to which this invention relates may suitably be produced, in the manner to be explained in detail below, from fibrous layers manufactured by well known air-laying processes.